Potassium permanganate (KMnO.sub.4) is produced from manganese ore in two stages. In the first stage, manganese dioxide is reacted with potassium hydroxide and oxygen to produce manganate VI. In the second stage, the manganate VI is electrolytically oxidized to potassium permanganate. These two oxidizing stages can be represented by the following equations: EQU MnO.sub.2 +2KOH+1/2O.sub.2 .fwdarw. K.sub.2 MnO.sub.4 +H.sub.2 O (1) EQU k.sub.2 mnO.sub.4 +H.sub.2 O .sup.electr. KMnO.sub.4 +KOH+1/2H.sub.2 .uparw.(2)
the plant operation for producing manganate VI involves the separation of the manganate VI from the ore residue. The impure K.sub.2 MnO.sub.4 containing the ore residue is extracted with dilute KOH solution to dissolve the manganate VI while leaving the ore residue undissolved. Prior to the present invention, the residue solids were separated from the extracted K.sub.2 MnO.sub.4, and the solids were disposed of as waste. Because of a high content of potassium they could not be disposed of as solid waste, such as in a landfill. The potassium content was objectionable for landfill purposes, since although the potassium was not readily soluble, it would gradually be released under the action of rain and ground water, resulting in undesirable contamination of the ground water. In prior practice, the semi-liquid waste was transferred to a lagoon in which the solids were permitted to settle, and the liquid to overflow into a stream, which was not satisfactory from the standpoint of avoiding stream pollution. A better method for preparing the manganate ore waste for disposal has therefore been needed, especially one which would reduce the potassium content of the waste. The process of the present invention accomplishes this purpose, while at the same time producing a dilute solution of KOH which can be returned to the manganate VI plant, and the remaining solid waste material can be disposed of in a landfill.
An industrial process for recovery and purification of manganate VI is described in U.S. Pat. No. 3,172,830. In that process, the manganate ore residue is separated from the extracted K.sub.2 MnO.sub.4 together with CaCO.sub.3 and Ca(OH).sub.2. The calcium carbonate and calcium hydroxide are present because of a causticizing treatment of the supernatant from the manganate VI crystallizer. More specifically, the supernatant from the crystallizer contains soluble potassium salts, principally potassium carbonate but also some potassium silicate and potassium aluminate. By treating the supernatant solution with calcium hydroxide, the potassium salts are converted to potassium hydroxide and the corresponding calcium salts are precipitated, including the calcium carbonate, silicate, and aluminate. Excess calcium hydroxide will also be precipitated, being thrown out of solution by the potassium hydroxide. The solubility of calcium hydroxide decreases rapidly as the concentration of potassium hydroxide increases.